A known thin film, heterojunction solar cell of such type which was widely regarded as viable, is described in U.S. Pat. No. 4,335,226, issued to Mickelsen et al on Jun. 15, 1982 and teaches the provision of a layer of cadmium sulfide (CdS) on an absorber layer of CIS to provide a cell of high efficiency.
Among the many documented attempts to improve such high efficiency cells are U.S. Pat. No. 4,611,091 issued to Choudary et al on Sep. 9, 1986, and U.S. Pat. No. 5,049,409 issued to Eberspacher et al on Sep. 3, 1991, the disclosures of which are incorporated by reference herein.
The Eberspacher patent teaches an improved method of selenization of the CIS semiconductor, while Choudary teaches the replacement of a majority of the cadmium sulfide layer by providing a layer of a transparent, substantially conducting, wide band gap, n-type semiconductor, such as CVD zinc oxide, on only a very thin layer of chemically deposited cadmium sulfide.
In essence, both the last two patents represent responses to the well recognized desirability of minimizing or eliminating the use of highly toxic materials such as hydrogen selenide and cadmium (sulfide) in large scale solar cell manufacturing processes.
The chemically deposited cadmium sulfide approach taught by Choudary resulted in higher quality junctions than those from non-chemical, (non-cadmium) deposition processes in providing lower junction saturation current and increased shunt resistance, such improvement believed to result from an etching and selective "cleaning" action on the heterojunction interface arising from the chemical dipping.
An attempt to eliminate completely the highly toxic cadmium sulfide from the cell by replacement with zinc sulfide (ZnS) deposited from a solution of zinc acetate (ZnAc.sub.2) dissolved in *ammonium hydroxide/ammonium sulfide (NH.sub.9 OH/(NH.sub.4).sub.2 S) was not successful. Although some zinc hydroxide (Zn(OH).sub.2), precipitate formed in the solution, no zinc could be detected on CIS dipped in that solution.
However, the discovery some years later, of CIS modules with cadmium sulfide layers chemically deposited from solution having actual thicknesses of less than 10 A.degree. but which, nevertheless, performed effectively, in spite of a belief that a minimum thickness of 100 A.degree. was necessary (in the Eberspacher cell), led to the speculation that the cadmium sulfide was in fact acting substantially only as an insulating barrier between the CIS and CVD zinc oxide and that, possibly, relatively non-toxic, insulating zinc oxide could perform the same function.
Although U.S. Pat. No. 4,612,411 issued to Weiting et al teaches a layer of CVD ZnO which is said to be highly resistive, this is clearly only relative to a top layer of ZnO of low resistivity (1.times.10.sup.-3 ohm-cm) as the resistivity is only, preferably, 0.2 ohm-cm while the minimum thickness is 100 angstrom. It is believed that the resistivity of the resistive zinc oxide layer deposited by the process of this invention is significantly greater than that obtained by the Weiting teaching while its thickness is believed to be very much less, and no greater than approximately 10 angstroms, as it is unable to be measured by SEM techniques.